1. Field of the Invention
The present invention generally relates to a method and system for making multi layer masks, and more specifically, to a method and system for creating more efficiently utilized multi-layer photomasks, to reduce the total photomask susbstrates required to produce integrated circuits.
2. Description of the Related Art
As semiconductor costs continue to increase, manufacturers are increasingly focused on maskset costs as a venue for cost reduction. Photomasks are templates representing a single layer of a multi-step assembly required for forming integrated circuits. Photomasks are used as projection templates where radiation is passed through a substantially transparent substrate, but in some regions, the radiation is absorbed by a patterned surface film. The pattern of the mask film is then propagated through a lithographic system and reproduced at an imaging plane of the lithographic system.
One of the most attractive methods for reducing maskset costs on low volume designs is through the use of Multi-Layer Reticles (MLR) or Multi-Layer Masks (MLM). These MLMs (or MLRs) include a single photomask substrate that contains a plurality of independent regions, each corresponding to one lithographic layer of integrated circuits. During usage, only one of the independent regions is exposed in the lithographic projection system at a time. Cost reductions are obtained by reducing total number of physical masks required to produce an integrated circuit.
Currently MLM photomasks are made by conventional photomask manufacturing methods, whereby a substrate is coated with a radiation sensitive organic polymer layer (photoresist), and in a direct-write lithographic system, the entire photomask, including multiple independent sub-regions, are patterned as latent images into the photoresist substantially at the same time, by the same photolithographic tool.
Photomasks can be “printed” by a number of different means. The two primary methods are by direct-write from either e-beam or laser tools that reproduce the pattern through physical manipulation of radiation. Laser print tools are generally quicker and require less image size and placement control than e-beam tools. Therefore cost of laser printing photomasks is substantially lower than e-beam printed photomasks.
Because of these limitations, and because a total number of available independent regions does not always correspond to a number of required masking layers, most MLM reticles are not fully spatially utilized, and, therefore, at least one of the potentially useable independent regions is not used for creating the integrated circuit.